1. Field of the Invention
The present invention relates to a stacked photovoltaic apparatus, and more particularly, it relates to a stacked photovoltaic apparatus including a plurality of power generation units.
2. Description of the Background Art
A stacked photovoltaic apparatus prepared by successively stacking a first power generation unit, an intermediate layer and a second power generation unit is known in general, as disclosed in Japanese Patent Laying-Open No. 2002-222972, for example. The aforementioned Japanese Patent Laying-Open No. 2002-222972 discloses a stacked photovoltaic apparatus comprising an intermediate layer of ZnO formed between a first power generation unit including a plurality of microcrystalline Si layers and a second power generation unit including a plurality of amorphous Si layers.
In the conventional stacked photovoltaic apparatus disclosed in the aforementioned Japanese Patent Laying-Open No. 2002-222972, the intermediate layer of ZnO is provided for reflecting light incident from the side of the second power generation unit to pass through the second power generation unit toward the side of the second power generation unit on the interface between the second power generation unit (amorphous Si layers) and the intermediate layer. In other words, the intermediate layer of ZnO in the aforementioned Japanese Patent Laying-Open No. 2002-222972 is provided for increasing the quantity of light incident upon the second power generation unit, thereby increasing the quantity of light photoelectrically converted in the second power generation unit.
In the conventional stacked photovoltaic apparatus disclosed in the aforementioned Japanese Patent Laying-Open No. 2002-222972, however, the difference (1.5) between the refractive index (2.0) of the ZnO layer constituting the intermediate layer and the refractive index (3.5) of the amorphous Si layers constituting the second power generation unit is so small that the quantity of light reflected on the interface between the intermediate layer and the second power generation unit is disadvantageously small despite the intermediate layer of ZnO. In the conventional stacked photovoltaic apparatus disclosed in the aforementioned Japanese Patent Laying-Open No. 2002-222972, therefore, the rate of increase of the light photoelectrically converted in the second power generation unit is so small that it is difficult to increase the quantity of short-circuit current despite the intermediate layer of ZnO. Consequently, it is difficult to improve the output characteristics of the stacked photovoltaic apparatus.